I ________ go to bed when I heard someone knocking at the door．

A.was about to

B.had just to

C.would

D.would like to

答案及解析： A

解析：

be about to…when…是一个常用句型，意为“正准备……这时……”。when作“这时”讲时，起并列连词的作用，连接两个并列分句。